Add a unique ID column to a Spark DataFrame. The Spark
monotonicallyIncreasingId function is used to produce these and is
guaranteed to produce unique, monotonically increasing ids; however, there
is no guarantee that these IDs will be sequential. The table is persisted
immediately after the column is generated, to ensure that the column is
stable -- otherwise, it can differ across new computations.
sdf_with_unique_id(x, id = "id")